Oksana Allegedly Told Dentist -- It Was Mel

7/12/2010 5:20 PM PDT BY TMZ STAFF

Oksana Grigorieva claims she told her dentist Mel Gibson knocked her teeth out the day after the alleged incident ... sources tell TMZ ... but there's a problem.

TMZ broke the story, Oksana claimed Mel knocked one tooth out and chipped another during an explosive argument on January 6. We're told Mel denies doing it.

The tape that Oksana secretly recorded with Mel during the showdown catches Mel saying, "You f**king deserved it."

Here's what's interesting about the dentist ... We're told the dentist never reported the incident to police. Under California law (Penal Code Section 11160), a health practitioner must report to police when the professional "reasonably suspects ... assaultive or abusive conduct" against a "spouse or cohabitant."

Sources tell us Mel and Oksana were indeed cohabitants on January 6.

The dentist could be a critical witness if Gibson is prosecuted for domestic violence. The failure to report could become a big issue.